High flow nasal cannula oxygen therapy (HFNC) is an emerging technique employed in the management of respiratory failures. HFNC provides through non occluding nasal prongs, a high flow heated and humidified medical gas, with a FiO2 ranging from 0.21 to 1.0. A high flow medical gas (up to 60 L/minute) is related at least to three main effects: the anatomical dead space wash out